I. Field of the Invention
This invention relates generally to X-ray imaging systems and related methods, and more particularly this invention relates to a system and related method of compensating for scattered X-rays and veiling glare fraction degrading of image contrast and sharpness.
II. Background Information
X-ray imaging systems provide non-destructive or in vivo images of an object such as a patient. An X-ray source irradiates an X-ray beam towards the object. The X-ray beam is attenuated and scattered by the tissues or elements of the object. A radiation detector detects the attenuated and scattered X-ray beam and converts it into an electrical signal indicating an intensity of the detected beam. This electrical signal may be displayed as a visual image on a TV monitor. The scattered radiation impinges on the detectors from a path outside a direct path from the X-ray source to the detectors. This scattered radiation is added to a primary or non-scattered radiation through a direct path from the source to the detectors. Consequently, that portion of the resultant image due to the scattered radiation obscures that portion of the resultant image due to the non-scattered radiation. Furthermore, fluoroscopic systems make the resultant radiation image unclear due to optical scatter known as veiling glare.
Conventionally, there are two well-known techniques to minimize the effects of scattered radiation. The first of these techniques is disclosed in U.S. Pat. No. 4,549,307 issued to Macovski. The assignee of this application has also filed the following applications which generally relate to this first technique: U.S. application Ser. No. 601,349, filed Apr. 14, 1984 by Kikuchi; U.S. application Ser. No. 673,792, filed Nov. 21, 1984 by Kikuchi et al.; U.S. application Ser. No. 719,168 filed Apr. 2, 1985 by Kikuchi et al.; U.S. application Ser. No. 792,855 filed Oct. 30, 1985 by Yamagata et al., now U.S. Pat. No. 4,741,009, issued Apr. 26, 1988; and U.S. application Ser. No. 857,050, filed Apr. 29, 1986 by Ema now U.S. Pat. No. 4,688,242.
In this first technique, actual scattered radiation is measured using an X-ray opaque dot or dots. The resultant scattered X-ray image at the locations of these dots is used to estimate the scattered X-ray image throughout the entire image. A radiation image is then obtained without the X-ray opaque dot or dots. This radiation image is corrected using the estimated scattered X-ray image for the entire image.
Using this first technique, it is necessary to irradiate additional X-rays towards the object to estimate a scattered X-ray image. This increases the X-ray dose delivered to the object; i.e., a patient. Furthermore, the scattered image between the X-ray opaque dots is estimated by interpolation, therefore the resultant scattered X-ray image for the entire image is not precise.
A second technique is taught by U.S. Pat. No. 4,599,742 issued to Kikuchi et al. The assignee of this application has also filed U.S. application. Ser. No. 575,549 on Jan. 31, 1984, now U.S. Pat. No. 4,653,080, issued Mar. 24, 1987 by Kikuchi et al., which generally relates to this second technique.
In this second technique, an acquired radiation image T is represented as follows: EQU T=S+P (1),
where S is a scatter-glare distribution and P is a primary or non-scatter distribution. This second technique depends on a theory that the scatter distribution S is approximated as follows: EQU S.apprxeq.cP.sup.n **PSF (2) EQU (** denotes two dimensional convolution operation),
where PSF is a scatter point spread function, and c and n are appropriate constants. Thus, this second technique teaches that the scatter distribution S is represented as a non-linear expression of the primary distribution P.
Furthermore, the equation (2) is approximated to solve the equation (1) practically as follows: EQU S.apprxeq.(aP+d)**PSF (3),
where a and d are appropriate constants defined so that a line represented by the equation (3) is tangent to a curve represented by the equation (2) at mean P in an P-S co-ordinate system.
Equation (1) is rewritten from equation (3) as follows: EQU T.apprxeq.aP**PSF+d**PSF+P (4).
The primary distribution P is obtained by solving equation (4) after acquiring a radiation image from the detectors.
In this second technique, however, the primary distribution P is obtained by directly solving equation (4) for each pixel. Thus, each pixel of the primary distribution P is calculated one pixel by one pixel, for example 512.times.512 times when the size of the acquired image is 512.times.512 pixels. Accordingly, this second technique takes a long time to obtain the primary distribution P, i.e., a scatter-free image.
Furthermore, since the primary distribution P includes a high-frequency component, an error is introduced when the primary distribution P is calculated from the equation (4).
An objective of the present invention is to provide an improved system and method for correcting for scattered X-rays.
In the case of using an image intensifier (I.I.), the present invention provides a system and method for correcting not only for scattered X-rays, but also for veiling-glare generated by optical scatter fraction from output phosphor.
Another object of the present invention is to provide a system and method for enabling a rapid correcting for scattered X-rays and/or veiling glare fraction.
Another object of the present invention is to provide a system and method for diminishing solution error in solving for the primary distribution of an image.